Many optoelectronic systems require a single-mode source of optical radiation. For example, optical communications systems typically require a source of optical radiation at wavelengths around 850 nm, 1310 nm or 1550 nm.
A laser diode used in a transmitter unit as part of a wavelength division multiplexing (WDM) fibre optic communications system may need to have a wavelength stability of within ±2 nm or better, depending on the number and spacing of different optical wavelengths to be carried by the fibre optic system. If wavelengths drift outside the specified ranges, then there will be cross-talk between adjacent channels in the WDM system.
However, the gain peak of a semiconductor laser may not coincide with a single optical mode and will, in general, change with both carrier density and temperature of the active semiconductor medium. For example, the Fabry-Perot (FP) gain profile of a typical III-V material semiconductor laser diode will have a maximum at a particular wavelength λ0FP, and the value of λ0FP will normally vary by an amount ΔλFP=0.5 nm/° C.
It is therefore often necessary to stabilize the operation of a semiconductor laser to suppress FP side modes, and to stabilize the wavelength of the optical radiation generated by the laser diode on a single generated mode.
One way of stabilizing the wavelength is to incorporate a grating structure with the laser diode. One example of a grating structure is a Distributed Feedback (DFB) grating. A DFB grating normally extends the length of a laser diode cavity, being integrated in an epitaxially grown cap layer above a waveguide layer and extending longitudinally between end facets that define the laser cavity. A DFB grating is normally a passive non-tuneable structure that stabilizes to some degree the laser wavelength, but without affording the ability to tune the laser wavelength
Another type of grating which can be used to tune the laser wavelength is a Distributed Bragg Reflector (DBR) structure. A DBR structure is usually incorporated in a waveguide that is separate from the waveguide containing the semiconductor lasing medium. A DBR structure may also be monolithically integrated with a laser diode structure on a common semiconductor substrate, or may be a discrete device that is integrated with a discrete laser device, for example being bonded to a common substrate.
In some applications, the temperature of the laser diode may also need to be stabilized, for example with an electrical heater if the laser diode is to be used at low operating temperatures (for example between 0° C. and −40° C.), or with a thermoelectric cooler if the laser diode is to be used at high operating temperatures (for example between 30° C. and 60° C.). This, however, adds complexity and cost to an optoelectronic component that includes such a temperature stabilized device. For these reasons it is desirable in low cost applications not to have temperature stabilization of the laser diode. The invention is particularly concerned with such low cost devices, but is also applicable to devices having temperature stabilization where such stabilization is not fully effective in all operating conditions to control the operating temperature of the laser diode, particularly at low temperatures of operation.
An advantage of incorporating a grating structure in an optoelectronic component having a laser diode is that the wavelength selected by the grating is more stable to temperature variations than the Fabry-Perot (FP) gain profile of the laser cavity itself. For example, in a device in which the FP profile may have a central peak λ0FP which varies by an amount ΔλFP=0.5 nm/° C., the grating wavelength λ0 G may vary by only ΔλG=0.08 nm/° C.
As is well-known, laser diode devices become more efficient at lower operating temperatures, which is to say that the threshold current for laser operation is reduced at lower operating temperatures. In a grating-stabilized laser diode, one facet of the laser cavity is made highly reflective and the other facet is made anti-reflective and functions as an output facet for the generated optical radiation. As compared with a cavity in which both facets have moderate or high reflectivity, this has the effect of raising the threshold current for the FP modes of operation. Compared with the FP modes of operation, the threshold of the single grating mode is substantially independent of the reduced facet reflectivity, since the grating stabilization is dependent on the interaction of the optical radiation on the multiple regularly spaced elements that form the grating. The threshold current of the single grating mode is therefore lower than any of the FP modes of operation. The result is that the anti-reflection coating has the effect of suppressing FP modes of operation relative to the single grating mode. The output optical radiation is then stabilized by the grating mode.
As the operating temperature of the laser diode and grating structure is changed, the grating wavelength ΔλG shifts at ΔλG=0.08 nm/° C. owing to a change in the refractive index of the lasing medium. Therefore, the grating wavelength ΔλG drops as the temperature drops. The threshold current of the single grating mode therefore also drops, but not as quickly as the threshold current of the suppressed FP modes of operation, because these have a higher wavelength/temperature coefficient ΔλFP=0.5 nm/° C. Therefore, at a sufficiently low temperature the threshold current of the suppressed FP modes will have dropped to the level of the grating threshold, and at this point the output of the laser diode will switch to operation at the shorter FP modes. This instability limits the low temperature operation of the laser diode.
Apart from temperature stabilization, a number of prior art approaches have been suggested to dealing with this problem. One is to decrease the output facet reflectivity and/or to extend the anti-reflection bandwidth to shorter wavelengths in order to further suppress the gain of the FP modes of operation at shorter wavelengths and hence lower operating temperatures. This approach suffers from the disadvantage that such higher performance anti-reflection coatings are more expensive to produce.
Another approach is described in U.S. Pat. No. 6,501,776 B1, in which the growth structure and hence band gaps of the semiconductor materials in the laser active medium and the grating structure are modified to change the values of ΔλFP and ΔλG so that the FP and grating gains do not converge at low temperatures. This, however, requires precise control of the growth of the semiconductor layers forming the active medium and grating structure, which adds to manufacturing cost, and also compromises the general device performance by reducing the overall efficiency of the device.
It is an object of the present invention to provide a more convenient optoelectronic device having a semiconductor laser and a grating structure for stabilizing the optical wavelength of optical radiation generated by the laser.